I realise it was perhaps not the most brilliant idea of mine to spark off a series of flippant remarks about what is in fact a potentially serious ontological issue.
I find the idea of two different personæ turning out to be one and the same biological person distinctly creepy.
(It happens rather brilliantly in Aldomovar's High Heels, remember? But Doppelgänger in fiction and film would require a separate thread...)